Caller-ID, also known as caller-identification (CID), or alternatively calling number identification (CNID), is a public telephone service provided over the plain old telephone service (POTS) lines that transmits a caller's number to the caller party's telephone equipment during the ringing signal, or when the call is being set up before the call is answered. Caller ID can optionally provide a name, either personal or company, with the calling telephone number, with the caller-ID information optionally being shown on the display of the called party's telephone. Caller ID on call waiting (CIDCW) operates in a similar manner with caller ID on call waiting displaying the number and optionally the name of an incoming caller when the called party is already on the phone.
Voicemail is a centralized system of managing telephone messages, typically for a large group of people, such as for a company in an enterprise environment. Operation of the system includes the playing of a greeting when a called party is not available, with the ability for the calling party to leave a message for the called party after the greeting has completed.
Any message left by the calling party can then be retrieved by the called party and played back at a later point in time. Additional functions for the called party include the ability to save a left message, forward a left message, respond to the message, callback the calling party, and the like.